A riddle about water heater expansion tank

Every year I shutoff the taps in the basement that leads out to the outdoor taps.
A couple of weeks ago I noticed that the relief valve on our hot water heater was leaking. We had the water heater rental firm out to the house 5 times to diagnose the leak and replace various parts during each visit. On their last visit they decided to replace the water heater entirely. Anytime we use the hot water, the hot water heater starts up and the relief valve leaks out approximately a cup of water. This is still happening with the new water heater installed.

Through trial and error I figured out that if I don't close the shutoff for one particular outdoor faucet, the water heater does not leak.
If I close the shutoff, the relief valve on the hot water heater leaks.

This may have been occurring every year but I just never noticed it since I rarely go into our furnace room.

Pipes don't expand much but water does with heat. If you have a closed system (a check valve or a PRV), then you need an expansion tank to give that water somewhere to go when it expands. Simple answer, as stated, get a new expansion tank, either replace the one you have, or add one and the problem should go away. There's often a 'weak link' in a house such as a toilet tank, or a poor seal on a faucet that leaks to relieve that pressure. It's also possible that the water company added a check valve, which made your previously open system now a closed one, or you replaced that weak link, and it no longer leaks to relieve the pressure.

That's an excellent point, Jim. The "thing on the line" I postulated above may be simply one slightly-drippy outdoor hose bib that nobody noticed was relieving the pressure. When the homeowner closed the ball valve, it precluded that path.

Not funny, it's downright stupid. This is exactly the symptoms of a closed system, and the expansion tank, or lack of one, is the first thing that should be suspected. I would bet you have either a pressure regulator valve or a check valve in your meter that is creating a closed system.

All great thoughts. Thanks. The previous owner may have included a check valve for a make-shift sprinkler system, but that would mean opening up the drywalled ceiling to get at it. ARGH!!!! Otherwise the expansion tank will be a good alternative. thanks again!

Well, if there's a sprinkler system out there, you definitely want a check valve unless you want bird-poo-and-fertilizer-water running back into your house water.

The issue as far as expansion goes is where the check valve is located. More and more today, on the asserted basis of preserving the integrity of the public water supply, utilities are installing or requiring a check valve at your meter. They call this a "closed" system because the pressure that builds up from water heating (more specifically, the water once it is delivered to you) can't just push back into the public water main, where that pressure would be distributed into and easily absorbed by that system because of its size. Instead, it must be absorbed by the relatively-small-volume of water-supply piping on your side of the meter, which it usually can't do. (Some but not all pressure reducing valves near your meter can have the same effect.)

If the check valve isn't between the major part of your system piping and the meter, it won't close your system, because the water is still free to push back into the main. So I don't think that sprinkler check valve is your issue. It's either a meter with a check valve built-in, a PRV, or a separate check valve near the meter. These are not bad things at all, and we are absolutely not suggesting that they be removed. They are just things that "close" the ability of your system to push water back into the public main, and thus require an expansion tank to keep the pressure from water heating from tripping your pressure relief valve or, in your case, apparently using the outside faucet connected to the line you are closing as a way to relieve the pressure.

One way to check whether our diagnosis is correct is to open the shutoff that has the effect, and then, when water is heating, go have a look outside and see if there's a little bit of water dripping out of that faucet (or anywhere else along that pipe beyond the shutoff, for that matter -- which would be Not So Good).

If you don't have an expansion tank, that's the way to go and it's no big deal to install. It's more-or-less the size of a basketball (well, maybe a tad bigger) and the plumbing involved isn't hard. Maybe even your water heater company would put one in for free, just to avoid having to keep replacing water heaters and water-heater parts. As Gary says, that's the first thing they should have thought of. All three of us (Jim Gary and I) -- who don't do water heaters for a living -- had exactly the same reaction.

Just a comment on the sprinkler system. I don't know Canadian laws regarding connecting a lawn sprinkler system to the domestic water supply, but in the states, a back flow prevention device is required. Although the law is enforced or not enforced to different degrees, common sense would tell you that while it may be a small risk, contamination can be pulled back into the domestic water supply from the lawn. Think the dog that takes a dump for example. Some cities are actually installing meters that have a check valve built in so that even household water can not reenter the water main. Where I live, I am absolutely required to have my back flow device re-certified by a licensed inspector annually. If I fail to do so, my water, all of it, is shut off. Other places aren't so picky, but they should be. Perhaps once every 5 or 6 years, the inspector will find a seal beginning to leak and will repair it. This device is more than a "check valve". You might want to research this and find out what Canada requires. A thermal expansion tank is installed in the incoming water supply line prior to it reaching the water heater. Cost for one is around $50 (US) and requires an ordinary tee in the line. Pretty simple and straight forward. I think the best way to tell if one is needed is to buy an inexpensive gauge that will screw on to a hot water faucet in the house. You connect it to the faucet, turn the water on, then draw hot water from another faucet until the heater kick on. Watch the gauge. If you need the expansion tank, you will be amazed at how fast the pressure rises to 150 psi. That's when the T/P opens to relieve the pressure.

The faucet you turn on and off is probably "leaking" so it drains off the expansion before it can build up. A faulty toilet fill valve would do the same thing. An "irrigation system check valve", (and it should be a backflow preventer/vacuum breaker NOT a check valve), SHOULD be on the irrigation system, NOT the house system.